Map onto Non-flat Earth
I've never done this, but I think you would get most accurate horizon distance, if you use UV Sphere with radius being average Earth's radius in your region and then work from there (remove all the surface area you are not going to need, subdivide faces at remaining region, apply uv texture), though its hard to tell if this method wouldn't be overkill or too inaccurate, depending on size of your region.

You can calculate Earth's radius at specific latitude with tool like https://rechneronline.de/earth-radius/

As for the textures - it is worth noting that no satellite photo is "flat" (like drawn map) and google maps doesn't display them on flat surface either - it is already mapped onto sphere, so you would have to map UVs "from view" (with viewport having as close settings as camera that took original image), again - maybe more work than needed, depending on region.